For every barrel of crude oil produced, about three to ten barrels of water is produced. In the oil and energy industry, water that is drawn from a formation is referred to as “produced water.” Produced water may be used interchangeably with “production water,” to refer to water separated from the production of stream and gas wells, including but not limited to tar sand wastewater, oil shale wastewater, water from steam assisted gravity drainage oil recovery process, and flowback water. Produced water is generated during oil production as a waste stream. In many instances, this waste stream can be seven or eight times greater than oil produced at any given oil field. Some of this water can be re-injected to the well for pressure maintenance, some is injected to deep well for final disposal in the case of proper aquifer conditions, and some is reclaimed for use as oilfield steam generator feed water. Large amount of water is typically needed for steam generation. Large amount of energy is needed to create steam from water. The produced water, which is not reinjected to the production well such as reclaimed water for steam generation, has to be treated. Produced water has distinctive characteristics due to organic and inorganic matters, potentially causing fouling and limiting steam generator reliability, and ultimately oil production.
The injection of steam for heavy oil recovery has become an important enhanced oil recovery (EOR) method. In EOR, high pressure steam is injected at a rate sufficient to heat the formation to reduce the oil viscosity and provide pressure to drive the oil toward the producing wells. For EOR, steam is normally produced in steam generators, with full steam makeup of water required to feed the generator. The feed water should be substantially free of iron and hardness, e.g., calcium and magnesium to prevent scale formation in the steam generator tubes or in the oil formation, causing plugging of downhole injection lines, causing increased pressure drop and increasing the power demand on pumps. Silica at high concentration can also pose a precipitation problem with scaling in steam generators and associated pipelines. Steam-generation quality water is water having less than 10 mg/L total organic carbon (TOC), less than 50 ppm silica and less than 1 ppm hardness as calcium carbonate. Since fresh water is not always available for EOR, the treatment of produced water in the oil recovery process becomes necessary.
In current practice, brine is formulated with de-oiled produced water which has significant hardness and residual oil impurities. This leads to inefficient regeneration of ion exchangers which results in hardness leakage during service cycle. Reverse osmosis (RO) is an option to provide water for making regenerant brine; however current reverse osmosis membranes are not able to operate at temperatures above 40-45° C. Therefore the sensible heat of produced water at very high temperatures cannot be utilized using current RO technology.
There exists a need for systems which better utilize sensible heat of produced water obtained in oil and gas production to improve energy efficiency of water treatment operations.